Nearly 190,000 people who were forced to leave their home under an evacuation order in California are now free to return — with the caution that the condition of the dam that imperils the area could still change quickly.

An evacuation order for communities near the Oroville Dam was reduced to an evacuation warning at 1 p.m. PT Tuesday, allowing residents to return, according to a statement from the Butte County Sheriff's Office.

"[H]owever all residents are advised to remain vigilant and prepared as conditions can rapidly change. People who have special needs or require extended time to evacuate should consider remaining evacuated," the statement said.

Officials in California were racing against the weather Tuesday, struggling to shore up the Oroville Dam's emergency spillway before more rains pummel the area and place the structure under even greater stress.

"I have determined this incident is of such severity and magnitude that continued effective response is beyond the capabilities of the state and affected local governments," Brown wrote.

The dam itself has not been damaged. But because the water levels are so high, the emergency spillway — which appears to be eroding — could unleash a wall of water onto the communities below if it collapses.

IF ANYBODY FROM OROVILLE AREA IS IN NEED FOR FOOD SHELTER, AND MEDICAL HELP PLEASE FEEL FREE TO HEAD TO TURLOCK SIKH TEMPLE. #OrovilleDam

Several state officials have told NBC News that, if the dam overflows, 1 million acre-feet of water could be released onto areas that are home to 188,000 people.

"We've never seen anything like this in modern times," a state water official told NBC News. "This is a worst-case scenario for any water management agency, a worst-case nightmare."

Officials have been "ramping up" a controlled release of water from Lake Oroville in order to drop the level below the range of what could threaten the area if the emergency spillway were to fail, Anita Chabria of the Sacramento Bee told Weather.com on Monday.

The release began last week in order to create more room in the reservoir behind the dam and alleviate some of the pressure on the spillway. The water level of Lake Oroville dropped on Monday, slightly easing fears of a catastrophic collapse.

But Oroville is bracing for rain on Wednesday that could add a strain to the spillway, and an additional problem looms.

Snow melt off of the Sierra Nevada mountains will continue to threaten Oroville into the spring, Chabria said.

As of Monday night, crews continued to drop sandbags and boulders to reinforce heavily eroded soil.

The possibility of disaster was raised 12 years ago by three environmental groups that filed a document with the Federal Energy Regulatory Commission that warmed that the emergency spillway might fail.

A spillway is a structure that allows a controlled release of water from a dam. The water is released so that it does not pour over the top of the dam — or even destroy it.